Crawfish Rock is one of the most remarkable and least-known dive sites in Victoria: a rock that shows at low tide in the middle of the North Arm of Western Port, off the north-western tip of French Island, whose sides plunge to 32 m on one face and 26 m on the French Island side into deep reefs, pinnacles, canyons and underwater channels. The deep water and the scouring tidal current keep sand and gravel from settling, and the result is a vertical garden of filter feeders of a richness found nowhere else in the bay – The Scuba Doctor describes a notably high invertebrate diversity and a unique and vulnerable marine community, home to threatened hydroids and other listed species, and the site is protected as a threatened community under Victoria's Flora and Fauna Guarantee Act. The walls are covered in sponges of every form and colour, soft corals, gorgonians, bryozoans, hydroid trees, ascidians and sea whips, with rock lobsters (the 'crawfish' of the name), blue devils, boarfish, morwong, wrasse and schooling sweep and trevally along the faces, and rays and draughtboard sharks on the sand at the foot.

It has been studied by marine scientists since the 1960s as the outstanding example of Western Port's deep-channel sponge communities, and its decline and partial recovery have tracked the health of the bay. Diving it is a serious exercise in tidal planning: the current is strong and reversing, slack lasts minutes rather than hours, the depth is technical-adjacent, and the visibility is estuarine. It belongs to experienced club divers with a capable boat, a skipper who understands Western Port, and a conservative plan; anchoring on the rock is prohibited.

For those who manage it, Crawfish Rock is a glimpse of the invertebrate wealth that made Western Port a Ramsar wetland, and one of the great hidden dives of the state.

Launch at Warneet (closest, on the northern shore), Hastings, Stony Point or Sandy Point/Somers and navigate the North Arm channel to the rock, which shows at low tide and is marked on charts – 20–40 minutes depending on the ramp. Because the rock is a protected community, anchor only on sand well away from it and drop divers at slack water to descend directly onto the reef; recover on SMBs. Tide times at Warneet Pier are the reference.

Private boats and clubs only – no charters run here.

Crawfish Rock stands in the main tidal channel of the North Arm and the current is strong and reversing – the whole of Western Port's northern basin drains past it – so the dive is possible only in the brief slack at the turn of the tide, preferably a neap tide, and must be timed from Warneet Pier tide tables with a margin. Wind is secondary because the site is enclosed, but strong southerlies raise a chop against the ebb. Visibility depends on rain in the catchment and on the tide: the last of the flood brings the cleanest water.

Winter dry spells give the best conditions; after heavy rain the North Arm is brown for a week.

Drop onto the rock at the last of the flood, descend the steep face on the deeper side to a planned maximum (20–25 m is ample), and work along the wall and through the canyons with the rock on one shoulder as the slack holds. Turn on time or gas, ascend the face to the shallows near the rock, and deploy an SMB for the boat as the ebb begins; be prepared for a drift pick-up.

Depths to 32 m on a strong tidal site; nitrox beneficial; some clubs dive it as a light-deco dive on the deeper face – plan conservatively and carry redundant gas and SMB. Threatened community listing prohibits anchoring on the reef and collecting.

No regular operator – Victorian dive clubs (notably the Victorian Sub-Aqua Group), marine scientists and experienced private boats; a handful of dives a year.